1. Field of the Invention
This invention relates to an automatic focus detecting apparatus capable of accomplishing focus detection, for example, in any area of the image field of a photographing apparatus by the Use of an area sensor and to the semiconductive photoelectric area sensor device thereof.
2. Related Background Art
A number of apparatuses capable of accomplishing focus detection at a plurality of points in the photographing image field have heretofore been proposed as automatic focus detecting apparatuses in cameras.
A sensor which is a photoelectric converting element used in an automatic focus detecting apparatus comprises a finite number of sensor arrays corresponding to focus detection points which are disposed on the same substrate, and specific examples of the construction thereof are described in detail, for example, in Japanese Laid-Open Patent Application Nos. 63-11906, 63-172209 and 1-271716.
FIG. 7 of the accompanying drawings shows the construction of the optical system of an automatic focus detecting apparatus according to the prior art. As shown in FIG. 7A, the optical system of the automatic focus detecting apparatus is comprised of a field lens 251, a multiaperture fleld mask 252, a secondary imaging lens 253 comprising two juxtaposed positive lenses, and a sensor chip 254 comprising a plurality of pairs of sensor arrays arranged. The multiaperture field mask 252 is provided at a location near the predetermined imaging plane of a photo-taking lens, not shown, and openings 252a, 252b and 252c in the multiaperture field mask 252 determined focus detection areas in the photographing image field. The secondary imaging lens 253 re-images a part of an object image limited by the opening 252a on the pair of sensor arrays 254a and 254b. Likewise, object images limited by the openings 252b and 252c are re-imaged on the pairs of sensor arrays 254c, 254d and 254e, 254f, respectively. The object image signals of the respective pairs of sensor arrays are read out as electrical signals and focus detection calculation is executed in a processing device.
The focus state of the photo-taking lens for the object in the focus detection fields of view determined by the openings 252a, 252b and 252c is detected in this manner. When the focus detection fields of view determined by the three openings 252a, 252b and 252c are applied to the photographing image field, they correspond to the positions of the focus detection fields of view 255L, 255C and 255R of the photographing image field 255 shown, for example, in FIG. 7B.
Where there are several focus detection fields of view and the positions of those fields of view are fixed, it is usual to discretely dispose sensor arrays on the sensor chip 254 corresponding to the positions of the respective detection fields of view as in this example, and provide a sensor driving circuit in the area between the sensor arrays on the sensor chip 254.
Where the number of the positions of focus detection is several like this, it is possible to discretely provide linear sensor arrays on one chip corresponding to the respective focus detection positions, as in the above-described sensor device 254. In the area between the discretely disposed linear sensor arrays, there can be provided a logic circuit and an analog circuit supporting the function of the linear sensor arrays and therefore, there can be constructed a device which has a high degree of integration.
However, if an attempt is made to make the disposition of the focus detection positions denser, this method has a limit and is inappropriate. The reason is that the linear sensor arrays each require temporary memory means for image analog information (which is necessary to serially output photoelectric charge information the accumulation of which has been completed in unison), a serial information transfer system, a shift register for effecting clocking for successively reading out information, etc., in addition to sensor picture elements, and these additional circuits require a much greater area than the sensor picture elements and thus, the number of sensor arrays which can be constructed in one chip is severely limited.
So, to dispose focus detection positions more densely than in the prior art, it is desirable to use a so-called area sensor in which sensor cells are two-dimensionally regularly arranged. In this case, the picture element information of a portion of the light receiving area of the area sensor is selectively calculated, whereby the focus state at a particular object position can be detected. In a camera containing an electronic image pickup device therein, such as a TV camera or a camera integrally provided with VTR, the use of both a sensor for image pickup and a sensor for focus detection is possible and therefore, not multipoint detection but focus detection using an area sensor has been put into practical use.
FIG. 22 of the accompanying drawings shows an example of it. A focus lens 107 is designed to be capable of being driven by a focus motor 108, and a solid state image pickup element 110 is attached to the center of a bimorph 109. The solid state image pickup element 110 is for photoelectrically converting optical image information formed by the focus lens 107, and usually has 100,000 to 500,000 picture elements and is directed by a video signal processing system, not shown, to output an image signal. The bimorph 109 is driven by an AC voltage from a bimorph driving circuit 111 and vibrates the solid state image pickup element 110 in the direction of the optic axis. The output signal of the solid state image pickup element 110 is connected to a blur detection circuit 112, which detects a front focus state (a state in which the lens is in focus to the front) or a rear focus state (a state in which the lens is in focus to the rear) by the vibration, rotates the focus motor 108 in a direction in which blur decreases, and drives the focus lens 107.
Generally, a main object which is the object of photographing and the background thereof coexist at a time in the entire image fleld of a picked-up image and therefore, in the blur detection circuit 112, the range of the image field which is the object of calculation detection must be restricted in some form, and in the prior art, such range is usually restricted to the central portion of the image field to thereby limit the control object range in advance. Alternatively, use is often made of a technique whereby a frame of a predetermined size is provided around the center of the image field and control is effected on a location which is highest in contrast within that range.
In the case of a focus detecting apparatus using a photoelectric area sensor in which picture elements are two-dimensionally arranged, the prior art has suffered from many problems as will hereinafter be described, and a satisfactory technique of arranging a number of focus detection points, and comparatively evaluating and controlling them has not yet been completely put into practical use.
A first problem is that the popular photoelectric area sensor for image pickup does not adopt a method of accessing to localized information at random. Generally in multipoint focus detection, it is necessary to data-calculate the image information of each detection point quickly and reflect the result of comparison and evaluation in focus adjustment control, and since this calculation process is executed by hardware using a microprocessor as a base or a digital circuit such as DSP, it is necessary to analog/digital (A/D) convert the image information and accumulate it in a digital memory. If the information of each focus detection point can be random-accessed, such data sampling will become remarkably easy in all points such as the hardware construction of the system, the capacity of the memory and the required speed of the A/D converter. In the prior-art area sensor, the function of the designated block for focus detection at random has not been sufficient and therefore, the construction of a satisfactory focus detecting function has been difficult. Particularly, a focus detecting apparatus of the phase difference type requires the photoelectric outputs of two corresponding optical images to be passed through different optical paths, and requires a sensor device for appropriately controlling and outputting two corresponding blocks separated from each other at synchronized timing. In an ordinary area sensor, necessary data must be taken out while the entire image field is read out at a uniformly high clock speed, and the timing of reading-out is limited in terms of hardware. This gives rise to a problem that in spite of a high-speed device being used, much time is required before the result of focus detection is obtained, and the ability of the system cannot be enhanced. Also, in some cases, it is desired to change the position of focus detection depending on the focal length of the photo-taking lens or the kind of the object, and it is an important factor that the focus detection point can be designated at random.
A second problem is that in an ordinary scene wherein the brightness or contrast of each focus detection point differs, optimal signal accumulation cannot be accomplished for each focus detection point. A person or a scene which becomes the object of image pickup in a photograph or video has a wide range of quantity of light and a main object is not always brightest. Cases where the scene which is the background is 10 to 100 times higher in brightness than the face of a person to be photographed occur frequently. Also, there is a point at which there is the regular reflection of the sun in the background and which is 1,000 times brighter than the main object. Accordingly, where an area sensor is applied to a system having a number of focus detection points, it is necessary that optimal accumulation control and the designation of amplification gain during reading-out can be accomplished for each focus detection point. An ordinary silicon photoelectric element used at normal temperature has a dynamic range of the order of only 100-1,000 and therefore, it is hardly be expected to secure sufficient S/N of each detection point by uniform control of the entire image field for a wide range of brightness fluctuation which becomes the object of photographing. The use of a prior-art area sensor results in optimal control being done for a location which is high in brightness, and this leads to the construction of such a system in which, independently of the photographer's intention, focusing is effected preferentially on an object of high brightness or an object of high contrast.
Further, if the disposition of focus detection points is made dense, the size of the picture elements of the photoelectric sensor becomes small and therefore, the quantity of light distributed to each focus detection point decreases and the performance of the low brightness side becomes bad. Accordingly, if the prior-art area sensor is used in a multipoint focus detection system, the deterioration of the low brightness limit performance and the determination of the auxiliary light effective distance during the projection of auxiliary light will be unavoidable.
A third problem is that an area sensor chip has a greater number of photoelectric conversion elements arranged on the surface thereof than in a line sensor and unsatisfactory picture elements are liable to occur correspondingly to the greater absolute number of picture elements. If no unsatisfactory picture element is allowed in using an area sensor, the yield will become very bad and this in turn will lead to an increase in the cost of the product and therefore, it is not realistic, and the area sensor must be used with a certain degree of dissatisfaction with the picture elements being allowed. However, if any unsatisfactory picture element exists Just in the area wherein the focus detecting process is carried out, an error will as a matter of course mix with the result of the process for that area and an accurate focus detecting operation cannot be ensured.